langevin: Split .ltm instead of .lang files

Usually, .ltm (testmodel) files are provided as multi-trajectory
files and .lang files as several single-trajectory files.
split: Keep comment lines in split data
langevin: Data concatenation for dLE trajectories

Concatenation of single-trajectory data in directory catdata
to multi-trajectory data with follower column.
langevin: Data concatenation for dLE fields